1. Field of the Invention
The present invention relates to resistance spot welders and in particular to precision direct current pulse welders for use in welding electronic circuitry and the like which are capable of delivering a constant amount of energy to each weld site.
2. Description of the Prior Art
In resistance spot welders, it is of prime importance to deliver the same amount of energy to each weld site in order to obtain uniform welds in a production setting. To accomplish this objective, numerous designs for welders and welding power supplies have been developed to deliver a precisely controlled pulse of voltage or current to the weld. This will accomplish the objective of uniform welds, provided each of the weld sites are identical.
In actual production situations, however, wire and the parts to which they are to be welded vary in thickness, surface finish, cleanliness and purity from weld to weld resulting in variations in impedance at each weld site. To further complicate matters, the contact area and pressure exerted by the welding electrodes vary with time during each weld cycle. All of these factors influence the weld schedule, requiring an adjustment in the voltage or current pulse, but in most instances, the instantaneous power is required to remain the same.
Many welders have been designed that allow the instantaneous power to vary, due to the above influences, but compensate for variations in weld site impedance by adjusting the duration of the weld energy pulse proportionately. This will work satisfactorily over a very narrow range of pulse amplitudes. However, half the power for twice the normal duration frequently does not produce a final welding temperture high enough to yield a strong weld, or may produce a metallurgically unacceptable weld.